Post by moodswing on Nov 24, 2008 16:47:54 GMT -5
Has anyone run into this problem? 
I am using a split-screen, with a bitmap at the top and text at the bottom. My problem is that on occasion, the SCPU incorrectly reads a VIC register. As in the following:
The problem goes away if I slow down to 1 MHz. It also goes away if I read $d016 or $d011 twice
or possibly wait in a loop
The faster my SCPU runs, the more likely the bug occurs. That is, if I mirror all memory, it doesn't occur as often as when I mirror one VIC bank and turn zero-page mirroring off.
I've had my SCPU for years and years and don't seem to have run into this problem before. Is my SCPU becoming faulty? Is my VIC becoming unreliable? Is there some SCPU timing issue I'm not aware of--following a VIC write immediately with a VIC read? I don't play games often, but when I have, I haven't noticed any problems, though I usually switch to 1 MHz.
I am using a split-screen, with a bitmap at the top and text at the bottom. My problem is that on occasion, the SCPU incorrectly reads a VIC register. As in the following:
...
stx $d021
lda $d016
ora #$10
sta $d016 ;occasionally becomes 38 columns
lda $d011
ora #$20
sta $d011 ;occasionally blanks the screen
...The problem goes away if I slow down to 1 MHz. It also goes away if I read $d016 or $d011 twice
...
stx $d021
lda $d016
lda $d016 ;read twice
ora #$10
sta $d016 ;stays 40 columns
lda $d011
lda $d011 ;read twice
ora #$20
sta $d011 ;no longer blanks
...or possibly wait in a loop
...
stx $d021
lda $d016
cmp $d016 ;debounce
bne *-6
ora #$10
sta $d016 ;stays 40 columns
lda $d011
cmp $d011 ;debounce
bne *-6
ora #$20
sta $d011 ;no longer blanks
...The faster my SCPU runs, the more likely the bug occurs. That is, if I mirror all memory, it doesn't occur as often as when I mirror one VIC bank and turn zero-page mirroring off.
I've had my SCPU for years and years and don't seem to have run into this problem before. Is my SCPU becoming faulty? Is my VIC becoming unreliable? Is there some SCPU timing issue I'm not aware of--following a VIC write immediately with a VIC read? I don't play games often, but when I have, I haven't noticed any problems, though I usually switch to 1 MHz.
